This invention relates to the production of gases from underground mineral formations, and more particularly to the enhanced production of natural gas or the components of natural gas from an underground coal formation using a strongly adsorbable fluid and a weakly adsorbable gas in combination to stimulate release of the desired gases.
Underground coal formations and other such carbon deposits contain natural gas components, such as the lower molecular weight hydrocarbons, due to effects of long term coalification. Coal generally has a low porosity, hence most of the coalbed gas is in the form of sorbate on the surfaces of the coal rather than being entrapped within the coal. The gas is present in the coal deposit in significant quantities; accordingly it is economically desirable to extract them for use as fuel and for other industrial purposes.
Coalbed gas is conventionally produced from underground coal deposits by pressure depletion. According to one technique for practicing this procedure, a well is drilled into the coal deposit and a suction is applied to the well withdraw the gas from the deposit. Unfortunately water gradually enters the coal deposit as the pressure in the deposit decreases, and as the water accumulates in the deposit, it hinders withdrawal of gas from the deposit. The drop in pressure as the process proceeds, and complications caused by the influx of water into the deposit, lead to a rapid decrease in the gas production rate and eventual abandonment of the effort after a relatively low recovery of the coalbed gas.
To avoid the difficulties of the above-described pressure depletion method attempts to recover gases from a coal deposit by injecting gaseous carbon dioxide into the deposit have been made. The carbon dioxide is injected into the coal deposit through an injection well which penetrates the deposit. The advantage of this procedure is that the carbon dioxide displaces the desired gas from the surfaces of the coal and sweeps it toward a production well which has also been drilled into the deposit, but at a distance from the injection well. Although this method affords a greater recovery of the coalbed gas than the pressure depletion method, it is prohibitively costly because large volumes of carbon dioxide are required to effect a reasonable recovery of the gas from the deposit.
It is also known to inject an inert gas, such as nitrogen or argon, into the coal deposit to force the coalbed gas from the coal deposit. This procedure is disclosed in U.S. Pat. 4,883,122. The method of recovery has the disadvantage that the inert gas is not adsorbed onto the coal; hence it does not easily desorb the coalbed gases. Consequently, although the inert gas does sweep some coalbed gas from the deposit, the inert gas is removed from the deposit with the coalbed gas. The presence of the inert gas in the coalbed gas removed from the deposit reduces its value as a fuel.
Because of the value of the coalbed gas, methods for the efficient recovery of coalbed gas from coal deposits which are free of the above-noted disadvantages of prior art recovery techniques are constantly sought. This invention provides such an improved method.